My current WIP has three main characters all of which will take turns being the viewpoint character. I wanted the narrative to reflect the personalities of each of the characters. That means varying my writing voice to match the character's. At first I thought - oh that will be simple.
I was wrong.
Luckily I have the the internet :)
Here are some great resources that I found for helping to develop individual character voice:
Character Voice Writing Exercises by Jennifer Jensen
This post gives some great exercises that not only help you develop character voice, but help you get to know your characters and their relationships to others. This has been very helpful.
Getting Into Character: Fiction Writing Exercises by Melissa Donovan
This post gives some tips on how to 'get into character' right before you write. I plan to use these when ever I switch narrators.
Character Development Exercises by Kathy Page
This post has a questionnaire that you can fill out 'in character.' It focuses mostly on the development of your character - which of course affects the voice.
Exercises for Story Writers
This page has some information about narrative and point of view.
How to develop character voice by Whitney Carter
More on developing deep rich characters.
I have not fully explored all the resources listed here - but what I have reviewed of them has helped in my writing.
Do you pay attention to character voice when you write? What are your tricks for 'getting into character'?